An electromechanical relay having a coil and relay contacts is often used to control the power level to a load by turning the power on and off repeatedly. Typically the modulation period is in the range of ten seconds to ten minutes. Many operations of the relay are required over the life of a product such as a microwave oven, in which a magnetron is often controlled by such a relay. Electric ranges also operate in this way.
The operational life of a power-switching relay is limited by contact erosion. One factor that affects the erosion of relay contacts is the phase angle on the AC line at which the contacts close. The ideal phase angle depends upon the type of load being switched. By applying power to the relay coil at the best phase on the AC cycle the life of the relay can be extended.
In the prior art a relay has been energized by an electronic controller in such a way as to control the time at which the relay contacts close with respect to AC variations of voltage in the load circuit. The time of closure of the relay contacts tends to vary from a desirable phase angle on the AC wave because of slow variations in the pull-in time that occur with heating of the relay coil, etc. In the prior art, a constant phase angle of closure time was obtained by sensing the time of closure and applying voltage to the relay coil at an earlier or later time to compensate for variations in the pull-in time.
Another factor affecting erosion is contact bouncing upon closure. A properly designed relay has a minimum bounce time when its nominal rate voltage is applied to its coil and the ambient temperature of the environment of the coil is about room temperature. Each relay has an ideal pull-in time for producing minimum bounce. When the coil is at higher temperatures, the resistance of the coil is higher and additional voltage is required to achieve the ideal pull-in time. Some previous control systems have adjusted the instant of application of power to the coil to compensate for variations in relay pull-in time, in order to achieve a desired phase angle of contact closure.